In natural gamma radiation logging, a tool capable of detecting gamma radiation in each respective one of several energy windows is passed through a selected borehole interval, and a record is made of the gamma rays detected within the respective windows. The gamma rays are emitted in the decay of subsurface materials such as thorium (Th), uranium (U) and potassium (K), each of which emits a characteristic spectrum resulting from the emission of one or more gamma ray at various energies. The tool output is converted to a log of the concentrations of Th, U, K at the respective borehole depth levels.
The Th, U, K log is important in searching for and exploiting underground resources because it is believed that these materials appear in nature with a discernible relationship to geology sand rock morphology. The log is particularly useful in the exploration for and exploitation of oil and gas resources because it is believed that the concentrations of Th, U, K taken individually or in combination are a good indication of previously unavailable information as to the presence, type and volume of shale or clay in the formations surrounding the borehole.
In practice, the nature of the logging process makes the detected spectra continuous, with poor energy resolution and poor counting statistics. Nevertheless, there are known techniques for usefully estimating and logging the Th, U, K concentrations.
The difficult measurement conditions in Th, U, K logging have been made even more difficult by the common use of borehole fluids (e.g. drilling mud) containing potassium chloride (KCI). These borehole fluids stabilize the borehole by reducing clay and shale hydration and provide various other benefits. However, the KCI in the borehole fluid emits its own gamma radiation which is merged with that of potassium in the undisturbed formations surrounding the borehole. Typically, the drilling and logging environment makes it impossible or impractical to measure the concentration of KCI in the mud at the time the borehole logging commences.
It is known that KCI in the mud filtrate has a significant effect in natural gamma radiation logging. See Cox, J. W. et al, "The Effect Of Potassium-Salt Muds On Gamma-Ray, and Spontaneous Potential Measurements," SPWLA 1976, and references cited therein. Additional uncertainties are introduced by the fact that relatively few gamma rays can be detected in the respective energy windows at a given borehole depth because the tool must move through the borehole at a sufficiently high speed to allow drilling or production activities to resume as soon as possible, and by the fact that the tool response changes as a function of borehole size.
U.S. Pat. No. 4,542,292 describes a method wherein is derived a log of gamma radiation detected in selected energy windows, (e.g. five windows) for a selected borehole interval, and converting it into a log of the selected materials, (e.g. Th, U, K) which is substantially corrected for at least one of: (i) a gamma ray emitter in the borehole fluid, e.g. potassium salts and (ii) a gamma ray attenuator in the borehole fluid, e.g. a strong attenuator such as barite and/or hematite. In a particular embodiment, the concentrations of the three materials (Th, U, K) are related through an empirically derived logging tool sensitivity matrix to five corrected window measurements. The method used for deriving the potassium correction in the '292 patent relies on the assumption that the spectrum of gamma-rays from the potassium in the borehole fluid is somewhat different from that in the undisturbed formations surrounding the borehole. These differences are in fact quite small and instability of the correction can result. The invention describes here makes no use of this assumption but provides a direct way of measuring the concentration of potassium in the mud.
A major problem with many of the correction methods is the assumption of the potassium chloride (KCI) concentration. Current operations depend on information from the personnel at the well site to learn the potassium concentration of the borehole mud. Often, the correct potassium concentration is not known therefore current operations use assumed values for the potassium concentration. These assumptions have been shown to be unreliable.
U.S. Pat. No. 5,120,955 describes a method and apparatus for correcting for borehole effects in logging natural gamma ray measurements wherein, for each depth in the borehole, a spectrum is made of the gamma rays detected in each of several energy channels and expressed as U=SY, where "Y" is the elemental yields and "S" is a composite spectra matrix made of individual standard spectra characteristic of a given borehole environment; yields "Y" are then calculated. A borehole correction factor is determined from the general relationship between the borehole parameters, the standard spectra and the measured spectra. The yields are then corrected for borehole effects by applying the correction factor. The correction is energy independent, for energies above a given threshold and is also independent from the radioactive elements.
The methods described in this patent are primarily for correcting the effect of mud weight and borehole size on the T, U, K measurement. Consequently, there is a need for an improved and simpler method for determining potassium gamma rays generated from borehole fluids for correcting natural gamma ray logs for borehole effects resulting from the borehole potassium gamma rays.